Genetics and Cellular Function

Transcription

1 Genetics and Cellular Function DNA replication and the cell cycle Mitosis Mitosis Mitosis: division of cells that results in daughter cells with the same the genetic information that the original cell had. 4-1 Diploid 2n Diploid 2 n 27-2 Chromosomes are made of DNA Madre Padre sister chromatids centromere duplicated chromosome (2 DNA double helices) A replicated chromosome consists of two sister chromatids Chromatid composed of DNA genes Almost all cells in body have homologous pairs of chromosomes homologous pairs in humans i.e., Total chromosomes Sister chromatids Fig. 8-5 Cell Cycle Most cells of body are in interphase - the non-dividing stage of life cycle Interphase is subdivided into: G 1 - cell performs normal physiological roles S - DNA is replicated in preparation for division G 2 - chromatin condenses prior to division Cell Cycle

2 Centriole Chromatids 1 Aster 2 Spindle fibers 3 Cleavage furrow Nuclear envelope re-forming 4 Chromatin Nucleolus Daughter cells in interphase 5/21/2012 Mitosis cell division in all body cells except the eggs and sperm Functions of mitosis growth of all tissues and organs after birth replacement of cells that die repair of damaged tissues 4 phases of mitosis prophase, metaphase, anaphase, telophase 4-8 Mitosis Prophase Chromosomes condense and nuclear envelope breaks down. Spindle fibers grow from centrioles. Centrioles migrate to opposite poles of cell. Metaphase Chromosomes lie along midline of cell. Some spindle fibers attach to kinetochores. Fibers of aster attach to plasma membrane. Anaphase s divide in two. Spindle fibers pull sister chromatids to opposite poles of cell. Each pole (future daughter cell) now has an identical set of genes. Telophase Chromosomes gather at each pole of cell. Chromatin decondenses. New nuclear envelope appears at each pole. New nucleoli appear in each nucleus. Mitotic spindle vanishes. Mitosis: Prophase chromosomes shorten and thicken coiling into compact rods chromosomes two chromatids per chromosome one molecule of DNA in each chromatid nuclear envelope disintegrates and releases chromosomes into the cytosol centrioles sprout elongated microtubules spindle fibers push centrioles apart as they grow pair of centrioles lie at each pole of the cell some spindle fibers grow toward chromosomes and attach to the kinetochore on each side of the centromere spindle fibers push chromosomes to line up along the midline of cell Mitosis: Prophase Mitosis: Metaphase Aster 1 Prophase Chromosomes condense and nuclear envelope breaks down. Spindle fibers grow from centrioles. 1 Centrioles migrate to opposite poles of cell. 2 Metaphase Chromosomes lie along midlineof cell. Some spindle fibers attach to kinetochores. Fibers of aster attach to plasma membrane. 2 Spindle fibers Ed Reschke 2

3 Mitosis: Anaphase Mitosis: Telophase chromatids cluster on each side of the cell 3 rough ER produces new nuclear envelope around each cluster 3 Anaphase s divide in two. Spindle fibers pull sister chromatids to opposite poles of cell. Each pole (future daughter cell) now has an identical set of genes. chromatids begin to uncoil and form chromatin mitotic spindle breaks up and vanishes Chromatids each nucleus forms nucleoli indicating it has already begun making RNA and preparing for protein synthesis Cytokinesis cytokinesis the division of cytoplasm into two cells telophase is the end of nuclear division but overlaps cytokinesis early traces of cytokinesis visible in anaphase achieved by motor protein myosin pulling on microfilaments of actin in the terminal web of cytoskeleton creates the cleavage furrow around the equator of cell cell eventually pinches in two 4-15 Figure 3.32: The stages of mitosis, p Centrioles Condensed Early mitotic Pair of Fragments of (two pairs) chromatin spindle centrioles nuclear envelope Aster Nucleolus Nuclear Plasma Chromosome, consisting envelope membrane of two sister chromatids microtubule Interphase Early prophase Late prophase Polar microtubules Spindle pole Figure 3.32: The stages of mitosis (continued), p Metaphase plate Contractile ring at cleavage furrow Nucleolus forming Spindle Daughter chromosomes Nuclear envelope forming Metaphase Anaphase Telophase and cytokinesis 3

4 Timing of Cell Division Cells divide when: they have enough cytoplasm for two daughter cells they have replicated their DNA adequate supply of nutrients are stimulated by growth factor chemical signals secreted by blood platelets, kidney cells, and other sources neighboring cells die, opening space in a tissue : division of cells that results in daughter cells with one-half of the genetic information that the original cell had. Cells stop dividing when: when nutrients or growth factors are withdrawn contact inhibition the cessation of cell division in response to contact with other cells 4-19 Diploid 2n Haploid n Pa Haploid n Ma Haploid n Cell division occurring in ovaries and testes to produce gametes (ova (egg) and sperm) Two divisional sequences Daughter cells have ½ the chromosomes the original cell had Junior = Zygote = diploid organism= 2n

5 In 1st division: homologous chromosomes pair along equator of cell rather than singly as in mitosis 1 member of homolog pair is pulled to each pole gives each daughter cell different chromosomes, consisting of 2 chromatids In 2nd division: each daughter divides; chromosomes split into 2 chromatids 1 goes to each new daughter cell Each daughter cell contains chromosomes Orginal mother cell had Genetic diversity & I (first division) II (second division) Genetic recombination occurs in prophase I 1. Crossing-over: Parts of one homologous chromosome are exchanged with its partner homolog 2. Independent assortment: the way chromosomes line up during metaphase is random Early prophase I Chromatin condenses to form visible chromosomes; each chromosome has 2 chromatids joined by a centromere. Mid- to late prophase I Homologous chromosomes form pairs called tetrads. Chromatids often break and exchange segments (crossing-over). Centrioles produce spindle fibers. Nuclear envelope disintegrates. Metaphase I Tetrads align on equatorial plane of cell with centromeres attached to spindle fibers. Anaphase I Homologous chromosomes separate and migrate to opposite poles of the cell. Telophase I New nuclear envelopes form around chromosomes; cell undergoes cytoplasmic division (cytokinesis). Each cell is now haploid. Chromosome Nucleus Centrioles Tetrad Crossing-over Chromatid Equatorial plane Cleavage furrow Prophase II Nuclear envelopes disintegrate again; chromosomes still consist of 2 chromatids. New spindle forms. Metaphase II Chromosomes align on equatorial plane. Anaphase II s divide; sister chromatids migrate to opposite poles of cell. Each chr omatid now constitutes a single-stranded chromosome. Telophase II New nuclear envelopes form around chromosomes; chromosomes uncoil and become less visible; cytoplasm divides. Final product is 4 haploid cells with single-stranded chromosomes Independent assortment The four possible chromosome arrangements at metaphase of meiosis I The eight possible sets of chromosomes after meiosis I Fig

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